1. Field of the Invention
The present invention relates to a method for controlling a pulse width modulator in the inverter of a PWM frequency converter provided with a voltage intermediate circuit and to a frequency converter provided with a pulse width modulator.
2. Description of Background Art
PWM frequency converters are generally used to feed electric motors when stepless control of the rotational speed of the motor is needed. FIG. 1 presents a PWM frequency converter connected to a 1-phase network to feed a three-phase load. It comprises a mains bridge 10 for rectifying the single-phase alternating voltage UL, N, of the supply network to produce a DC intermediate-circuit direct voltage UDC1 and a load bridge (inverter) 11 for inverting the DC intermediate-circuit direct voltage to produce a variable-frequency three-phase alternating voltage UR, US, UT, which can be used e.g. to feed a three-phase motor 13. The load bridge is a full-wave bridge with a control unit 12 controlling the semiconductor switches V11–V16 of each phase, each of the switches being connected in inverse-parallel with a free-wheeling diode D11–D16. The mains bridge 10 may be an uncontrolled full-wave bridge with upper and lower arm diodes D1–D4 connected to the phase and free-wheeling arms. As is well known, to limit mains current harmonics, PWM frequency converters use either an AC inductor LAC as presented in the figure or a DC inductor connected between the mains bridge and the intermediate-circuit capacitor CDC.
A PWM inverter is used to produce a motor supply voltage whose amplitude and frequency can be adjusted independently of each other. The motor may be a cage induction motor or e.g. a permanent-magnet or separately excited synchronous motor. As is known, the control signals for controlling the semiconductor switches V11–V16 of the inverter bridge are generated in a PWM modulator by using e.g. sine-triangle comparison or vector modulation, such as space vector PWM. Space Vector PWM (SVPWM) is a pulse width modulation method especially well suited for digital implementation of the modulation of the inverter of a frequency converter provided with a voltage intermediate circuit, wherein the switching times for the switch positions of the inverter bridge are generally calculated by software. From the switching vectors used during a switching cycle, an output voltage space vector consistent with the reference value is formed as an average value.
Normally, the switching references for the inverter are generated utilizing a measured intermediate-circuit voltage value Udc1, so the motor supply voltage is consistent with the reference value regardless of small variations in the intermediate-circuit voltage.
In frequency converters fed from a three-phase supply, the direct voltage circuit can be implemented by controlling the frequency converter e.g. according to patent specification FI 111201, in which case the capacitor in the direct voltage circuit will not serve as an intermediate energy storage and its voltage in the loaded state will follow the rectified supply network voltage. Thus, the capacitor is not needed to smooth the intermediate-circuit direct voltage, so it is possible to use capacitor of a considerably lower rating than conventional intermediate-circuit capacitors (typically only about 1% of the value of a conventional capacitor).